Perhaps that is right for Jirachi, but Jirachi is still tanky enough to take hits, especially with the Z happy hour boost. Zen headbutt 2 HIT KO defensive Celesteela(not 1HKO unfortunately) but celesteela isn't able to do anything simply because it was flinched before doing so.
Also, Z happy hour Jirachi has a base attack of 150 with the boosts and it is bulky enough to bring down at least 2 pokemon from the enemy team. Z happy hour Jirachi is so fast that it outspeeds many non scarf users, and if it is against a scarf user, it is bulky enough to so it won't be revenge killed easily. He is super difficult to revenge kill and my jirachi wrecks the enemy team so bad that they forfeit most of the time in my experience anyways. He has the same typing to M-Metagross, making it weak to only, fire, dark, and ground attacks. Z-Happy jirachi is definitely not as good as M-Metagross, but against the pokemon in A-, like Tapu Fini and Tapu Koko, It will 2 hit KO with iron head, Hit Volcarona and flinch him before doing anything, tank a fire blast from Volcorana, and many more. Looking at the mons in B-, I'm not sure if they can compete(for the exception of Chansey). Tapu bulu's Wood happer does nothing and he can 2 hit or 1 hit KO Tapu bulu with Iron Head. Jirachi can beat Nillego with two iron Heads. It deserves at least a nomination.
As for Chansey, it doesn't struggle badly with physical attacker once he land his thunder wave. Afterwards, he can poison them and take hits like its nothing and follow up with a fancy seismic toss.
Sure, Chansey's physical bulk isn't the best but it can tank hit because of its insane HP.
Chansey doesn't die easily, even with Physical attackers, so he can setup hazards without worrying that he will be 1hit KOED, because it is almost impossible.
Poison and paralysis? At the same time? Wat?
Take hits like it's nothing... Somehow I doubt that. Yes, it checks special attackers. But few special attackers are gonna stay in anyway.
It's not burning them, so even if the physical attackers in question are paralyzed, they're still gonna rape Chansey.
You're missing something else: Chansey is not very splashable at all. It fits on stall teams and stall teams only. It's a huge momentum sink on any other team.
Anyhow Chansey is now in A-, so you got your wish.
So it's interesting that you bring up Volcarona in your Jirachi argument. Volcarona has an interesting ability called Flame Body, which can potentially burn things that make contact with it. Then all your boosts go to crap.
Also you still have no calcs.
As for the 150 base Attack, it doesn't help with the fact that you get walled to the moon and back by things like...
-Mega Scizor (which can set up)
-Volcarona (which too, can set up)
-Heatran (assuming you don't run Drain Punch)
-other bulky Steels and Fires.
Yes, it beats Fairies and Nihilego. Cool. The sheer amount of support that it needs to even set up, however, makes me very much reluctant to see Jirachi rise because of its Happy Hour set, of all things.
Edit: I don't know what kind of crap calculator you're using for the 2HKO on defensive Celesteela, but that's a no.
+1 236 Atk Jirachi Drain Punch vs. 252 HP / 4 Def Celesteela: 98-116 (24.6 - 29.1%) -- possible 5HKO after Leftovers recovery
+1 236 Atk Jirachi Zen Headbutt vs. 252 HP / 4 Def Celesteela: 78-93 (19.5 - 23.3%) -- possible 6HKO after Leftovers recovery
And if you're crazy enough to run Thunderbolt...
+1 0 SpA Jirachi Thunderbolt vs. 252 HP / 252+ SpD Celesteela: 138-164 (34.6 - 41.2%) -- 69.5% chance to 3HKO after Leftovers recovery
That's the SPECIALLY DEFENSIVE set.
Edit 2: Oh, welcome to Smogon by the way! I would suggest you lurk about a bit or get tutoring before you commence posting.